1. Field of the Invention
This invention relates to intraocular lenses and methods of affixing fixation members to intraocular lenses. More specifically, this invention relates to the use of a neodymium yttrium aluminum garnet (Nd YAG) laser for the staking of fixation members to the optic bodies of intraocular lenses. These laser-staked lenses are substantially free of heat-induced optical deformities and provide a strong bond at the junction between the optic body and the fixation member without gaps for potential accumulation of bio-burden.
2. Description of the Related Art
It has become a routine practice to implant intraocular lenses after a patient has had cataract surgery to remove the natural human lens. These lenses generally include a plastic lens optical body which is designed to focus light, and a fixation member for holding the lens in place in the eye.
Fixation members vary widely in design and include iris clips of various configurations and haptics of various shapes, including the now-popular "J-shaped" haptics. Haptics now often comprise a pair of opposed outwardly disposed thin loops for holding the lens in place in the posterior chamber of the eye.
Fixation members may be fabricated of a suitable biocompatible material, such as platinum-iridium wire, or plastic loops made of a thermoplastic material, such as polypropylene, or a thermosetting material, such as polymethylmethacrylate.
U.S. Pat. No. 4,104,339 states that extracapsular lenses normally have one pair of opposed outwardly disposed loops, generally of metal wire such as platinum-iridium, and which are designated "posterior loops" because the ends are attached into and through the rear face or base of the lens. The four ends of the two loops are generally arranged in a square pattern at distances of about 1 mm from the center line of the lens, outside the immediate line of vision of the wearer. Loop wires are of 0.15 to 0.20 mm diameter and extend downwardly from the base of the lens.
The '339 patent claims as its invention a method of connecting fixation members of thin metal wire to a thermoplastic intraocular lens body. In this method, loops are positioned in a chuck and are heated by passing electricity through the chuck's clamp jaws to cause inductive heating of the wire material to slightly above the melting point of the plastic material of the lens body. The ends of these heated wire loops are then pushed into the peripheral edge of the lens body. Upon cooling, the ends of the wire loops are said to be firmly secured to the lens body. When the loops are fabricated of plastic line, holes are first drilled into the side walls of the lens body, less than halfway through the lens body. The ends of the loop are then inserted into these holes in the lens body and are heat-fused at a point along the inserted length to the lens material. This heating is accomplished by pushing a thin probe inductively heated to above the melting point of the lens material through the base of the lens body until it contacts the loop line material. It is represented that only the material in the immediate vicinity of the probe entry point is melted and the resulting heat-deformed ring is said to be about three times the diameter of the probe. Therefore, this method of attachment by use of a heated probe is akin to spot welding so that fusion of the loop line ends and the lens body only takes place in the vicinity of the point of contact with the heated probe. Consequently, it may be expected that the polymeric loop ends would not be as tightly attached to the lens body as when the entire inserted loop end is melted, caused to flow, and thereby bonded and attached to the lens body. Further, it might be expected that there would be gaps between the inserted polymeric loop line ends and the walls of the holes in the lens body. These gaps may be expected to provide a space for microbial contamination or "bio-burden" and prevent ready sterilization of the heat staked intraocular lens.
U.S. Pat. No. 3,994,027 is directed to a pre-pupillary lens of polymethylmethacrylate having a planar and a convex surface and having a pair of posterior loops embedded and fused into the optic portion. In a second embodiment, the lens includes a pair of anterior loops with their ends inserted in a set of two parallel bores which are disposed in a plane between the planar and convex surfaces of the lens. The loops are formed of platinum-iridium wire and are heated to a temperature in the range of 125-200.degree. C. before being embedded into the lens optic. The '027 patent also discloses the use of "an electronic bonder" to apply to a "heat tack" to the surface of a lens, adjacent the borehole into which the loops fit. It is represented that this heat tack fuses the ends of the loop to the lens body.
U.S. Pat. No. 4,786,445 is directed to a method of affixing fixation members to an intraocular lens by using laser energy to cause melting of inserted ends of the fixation member. In this method, a hole is drilled in the periphery of the optical body and a shoulder is formed within the hole near its inner end. One end of a fixation member is fitted into this hole and laser energy in the near infrared band is applied to cause the inserted end of the fixation member to melt and flow behind the shoulder. Upon cooling, the flowable portion of the fixation member hardens within the shoulder of the hole and forms a mechanical interlock which resists rotation and withdrawal of the fixation member from the cavity. In other embodiments the hole is tapped and provided with screw threads so that when the inserted end of the fixation member melts and flows, it flows into the threading. Upon hardening, the mechanical interlock between the threads and the flowed portion of the inserted end of the fixation member resists withdrawal of the fixation member.
U.S. Pat. No. 4,843,209 is directed to an apparatus for laser heat staking of intraocular lenses. The method uses a computer-controlled Nd YAG laser with a helium neon (HeNe) aiming laser which feeds a fiber optic multiplexer controller. As a result, several workstations can be operated at one time. The workstations have x-y work tables fitted with carousel holders into which intraocular lenses are placed for staking. A laser stylus is manipulated into position above the junction of the optic and the fixation member. The laser is then energized, fusing the inserted ends of the fixation member in the hole of the optic. It is represented that, as the haptic is lased, the haptic material within the optic swells. This, in combination with the internal fusion and melting, effectively seals the majority of the haptic hole to produce an intraocular lens with reduced potential of bio-burden. It is suggested that the optic does not melt because the polymethylmethacrylate material is about 95% transparent to YAG laser energy as long as its power density does not exceed the plastic's threshold temperature. It is further represented that a pigmented haptic loop, which will absorb more laser energy, and which fuses or heats faster, can also be used. It is represented that a continuous wave Nd YAG laser beam of 25 watts power applied for 1.5 seconds is sufficient to bond haptic to optic. However, in applying the method as explained in the '209 patent, it has been found in practice that blemishes are formed on the optic surface at the point of laser contact and the fixation members are not firmly attached.
What is yet needed is a method of staking intraocular lenses that is simple, reliable, and relatively inexpensive, that allows the staking of PMMA fixation elements to PMMA optics, that avoids rotation and misalignment of the fixation elements during staking, that reduces gaps between the fixation element and the borehole in the lens optic for reducing the potential for bio-burden accumulation, that attaches the fixation element firmly to the optic, and that does not adversely affect the optical properties of the lens in the vicinity of the staking process.